Video cameras that utilize exclusively solid state components rather than a mechanical movement of lenses and other optical components to pan, tilt, and zoom have numerous economic and functional advantages. An electronic pan, tilt, zoom (“EPTZ”) camera is small, fast, inexpensive, quiet, reliable, durable, easy to manufacture and easy to upgrade. Cameras with electronic EPTZ features can be used in, for example, cameras incorporated in set-top boxes for video conferencing, in surveillance cameras, camcorders and digital still cameras, among other applications.
A digital EPTZ camera typically has three main independent components: an imager section, a sharpening filter, and a scaling circuit or “scaler”. The imager section typically includes a lens and a solid-state imager chip, such as a charge-coupled device (CCD) or a metal-oxide-on-silicon (MOS) type chip. The imager chip includes a plurality of photosensitive areas (that is, picture elements or “pixels”) arranged as a two-dimensional array which is scanned either interlaced or progressively, as is known in the art. Medium resolution imager chips which perform satisfactorily in video set-top applications have, for example, 640 pixels horizontally and 480 pixels vertically. The imager section preferably includes an analog-to digital section to provide digital serial video information. Imager sections and imager chips of this type are well known in the art.
Conventional EPTZ cameras sharpen the image before the image is scaled by the scaler. Virtually all modern imaging systems utilize a standard Laplacian sharpening filter to reduce blurring that results from hardware limitations. The standard Laplacian filter is known in the art and applies a high-pass function to both horizontal and vertical image axes. High-pass filters, however, tend to amplify noise and introduce high-frequency artifacts into the image, such as undershoots and overshoots, which are perceived by the viewer as “halos”.
The scaling circuit electronically adapts or “scales” the number of input pixels to match a predetermined output format. For example, video conferencing signals are typically transmitted in the Common Interface Format (CIF) of 352 pixels horizontally and 288 pixels vertically. The imager section described above outputs 640 pixels horizontally and 480 pixels vertically. The scaler has to convert the image to CIF format by “down-sampling”, in this example by a factor of 1.83 horizontally and a factor of 1.67 vertically. The scaler generates the respective new horizontal and vertical CIF addresses and computes new video information, e.g., new luminance (Y) and chrominance (C) values associated with the new addresses. Consequently, the scaler can also be used to perform the pan/tilt and zoom functions in an EPTZ camera.
Conventional EPTZ cameras typically do not sharpen the images after scaling. In particular, zoomed-in images tend to appear blurred. The visual appearance of blurry images is most effectively enhanced by sharpening “edges” in the image. An edge can be defined as a linear arrangement of pixels where the luminance (Y) signal changes significantly in a direction perpendicular to the direction of the linear pixel arrangement.
It is therefore an object of the present invention to provide an adaptive edge sharpening apparatus and method which sharpens an image without significantly amplifying noise.
It is a further object of the present invention to adapt the parameters used in the sharpening algorithm to the scaling ratio of the image. It is still another object of the invention to prevent overshoot and undershoot of the sharpened pixel values.